1. Field of the Invention
The present invention relates to an output stage for driving a load and, more particularly, but not by way of limitation, to a low distortion output stage for driving a low impedance load.
2. Description of the Related Art
Audio chips presently enable personal computers, compact disk players, and other portable audio devices to execute high quality, low power audio applications. Audio chips usually comprise digital circuitry which occupies approximately 75-80% of the audio chip's silicon space and analog circuitry which occupies the remaining 20-25%. Typically, the analog circuitry comprises an analog-to-digital converter, digital-to-analog converter and some output amplifiers. The analog circuitry converts an analog audio input signal into a digital format suitable for the digital circuitry to process. Also, the analog circuitry converts the digital signals back into an analog format suitable to drive a low impedance load, such as a speaker. The digital circuitry occupies the majority of the silicon area and typically performs digital signal processing, such as filtering, noise shaping, and synthesizing, on the converted analog signals. The main function of these audio chips is to implement an entire audio system on one piece of silicon.
The conventional analog circuitry utilized to implement high quality audio chips comprises a differential operational amplifier (op-amp) having at least one gain stage cascaded to an output stage. Because the present trend in analog circuitry design utilizes smaller power supplies, the differential op-amp should be capable of operating from a smaller power supply, yet continue to provide the output range performance found in higher voltage applications. To satisfy this requirement, the output stage of the differential op-amp must have very low distortion so that it is capable of swinging nearly rail-to-rail to drive the low impedance load.
FIG. 1 illustrates output stage 200 of U.S. Pat. No. 5,198,782, entitled "Low Distortion Amplifier Output Stage for DAC", issued Mar. 30, 1993 to Jeffrey W. Scott. Referring to FIG. 1, output stage 200 comprises source follower transistor 266, sourcing transistor 284, sinking transistor 286, feedback transistors 290, 288, and 292, and constant current source transistors 280 and 282.
Specifically, transistor 266 is configured as a unity gain source follower and comprises: 1) a gate for receiving an analog input signal (V.sub.IN); 2) a source connected to the output of output stage 200 (i.e., node 260); and 3) a drain connected to node 268. The voltage at node 260 follows V.sub.IN minus the gate-to-source voltage drop (V.sub.GS) across transistor 266. In other words, the output of output stage 200 follows the analog input signal V.sub.IN minus V.sub.GS.
P-channel transistor 280 includes a source which is connected to a DC power supply V.sub.DDA and a drain which is connected to node 268. A biasing circuit (not shown) biases the gate of transistor 280 such that transistor 280 remains in its saturation mode. Consequently, transistor 280 functions as a constant current source to provide a constant drain current to transistor 266. N-channel transistor 282 includes a source which is connected to ground V.sub.SSA and a drain which is connected to node 260. A biasing circuit biases the gate of transistor 282 such that transistor 282 remains in its saturation mode to sink a constant current between the source of transistor 266 (i.e., node 260) and ground V.sub.SSA. Accordingly, because the current through transistors 280 and 282 remains constant, the current through transistor 266 remains constant to ensure that changes in V.sub.IN will be reflected at output node 260 and, thus, load 264.
Sourcing and sinking transistors 284 and 286 source and sink current, respectively, to maintain the appropriate voltage (i.e., V.sub.IN minus V.sub.GS) at node 260. Specifically, during negative swings of V.sub.IN, sinking transistor 286 sinks more current to drop the voltage at node 260. Conversely, during positive swings of V.sub.IN, sourcing transistor 284 sources more current to increase the voltage at node 260.
Unfortunately, as previously described, the voltage at node 260 cannot entirely follow (i.e., equal) V.sub.IN because of the gate-to-source voltage drop (V.sub.GS) across transistor 266. In fact, the output voltage at node 260 may be, illustratively, between 0.7 to 1.5 volts less than V.sub.IN. Therefore, output stage 200 cannot drive node 260 and, thus, load 264 near the upper rail voltage (i.e., V.sub.DDA). This voltage drop V.sub.GS is particularly significant and disadvantageous when small DC power supplies are employed. Illustratively, if V.sub.IN =2.7 volts, the output voltage at node 260 could be as low as 1.2 volts (i.e., 2.7-1.5=1.2). To compensate for the V.sub.GS drop across transistor 266, the gain stage or stages which precede output stage 200 must be capable of swinging a full V.sub.GS higher than output stage 200, placing significant design constraints on these gain stages. Therefore, output stage 200 is undesirable for use in low voltage and/or high signal swing applications because its output swing cannot approach the rail voltages.
Accordingly, a market demands exists for a low distortion output stage which is capable of swinging nearly rail-to-rail. This output stage would be particularly well suited for low voltage applications that require the output stage to operate from a low supply voltage, yet provide the output range performance of higher voltage applications.